High resolution x-ray imaging systems, also known as X-ray imaging microscopes (“XRM”), provide high-resolution/high magnification, non-destructive imaging of internal structures in samples for a variety of industrial and research applications, such as materials science, clinical research, and failure analysis to list a few examples. XRMs provide the ability to visualize features in samples without the need to cut and slice the samples. XRMs are part of the field of x-ray microscopy.
XRMs are often used to perform computed tomography (“CT”) scans of samples. CT scanning is the process of generating three dimensional tomographic volumes of the samples from a series of projections at different angles. XRMs often present these tomographic volumes in two-dimensional, cross-sectional images or “slices” of the three dimensional tomographic volume data set. The tomographic volumes are generated from the projection data using software reconstruction algorithms based on back-projection and other image processing techniques to reveal and analyze features within the samples.
Operators select scanning parameters, such as x-ray energy value, exposure time, and filter settings, and direct the XRM to perform a CT scanning “run.” For each run, the operator or an automatic loader installs the sample between an x-ray source and an x-ray detector system, and exposes the sample to a beam of x-rays. The XRM rotates the sample in the x-ray beam, and its detector system detects the x-rays that are transmitted through and modulated by the sample at each rotation angle.
During a run, the sample absorbs or scatters some of the x-rays before passing through to the x-ray detector system. The x-ray detector system receives the attenuated photon flux of x-rays that pass through and are spatially modulated by the sample. The detector system creates an image representation, in pixels, of the x-ray photons that react with the detector system. X-ray absorption increases with sample density and thickness, and is also generally higher for elements within the sample that have a higher atomic number (“Z”) in the periodic table.
Operators use standard operating procedures and best known methods (“BKM”) for the selection of the optimum “run” conditions. BKMs are written instructions for workflows that are written instructions for workflows that help the operator determine the optimum x-ray source voltage settings, beam pre-filter and detector settings associated with a particular sample. The resulting three-dimensional image representation of the sample after processing is also known as a reconstructed tomographic volume data set.
Operators typically operate an XRM using software control. For each scanning run, also known as a single energy scan, operators set the scanning parameters. Scanning parameters include variables such as the x-ray source voltage setting, exposure time, and source filter settings.
A related technology of XRM is x-ray fluorescence (“XRF”) microscopy. XRF microscopy utilizes x-rays differently than does XRM. Operators use the secondary x-ray energy emission associated with XRF, or fluorescence, to uniquely identify individual atomic elements (“Z”) within the sample.
In XRM, the contrast mechanism for attenuation in the sample has two principal components in the x-ray energy range of interest called the photoelectric absorption component and the Compton scattering component. In the photoelectric absorption process, an x-ray is absorbed completely by a bound electron of an atom and ejects this electron from the atom. In the Compton scattering process, the incident x-ray loses part of its energy and gets redirected by scattering off an electron. The effects of both components contribute to the image in an XRM arising from attenuation of the illuminating x-ray beam.
The relative strength of the photoelectric absorption and Compton scattering processes is a strong function of incident x-ray energy and the atomic number Z of the atom that interacts with the x-ray. The absorption due to the photoelectric effect generally dominates at lower energies and decays in strength inversely with the fourth power of the x-ray energy. The absorption due to the Compton scattering effect becomes dominant at higher energies and has a much slower decay with x-ray energy (inversely with first power of energy).
The transition point between photoelectric and Compton scattering absorption is referred to as the “knee,” where the absorption changes from decaying inversely with the fourth power of the energy to the first power. This knee is a characteristic of the atomic number Z of the atom and increases with increasing Z.